1. Technical Field
The present disclosure relates to lasers and more specifically to adjusting the central frequency of a laser while maintaining frequency stabilization to an external reference.
2. Introduction
In the field of communications between multiple spacecraft, lasers currently provide one possible communications medium. Lasers known in the art are unable to frequency stabilize and still vary the central frequency so as to achieve a heterodyne beat frequency within the radio frequency (RF) bandwidth of a photodetector and transfer the frequency reference from a local on-board source, such as a Fabry-Perot cavity, to an non-local source. One example of a non-local source for one spacecraft is a laser beam from a second spacecraft.
One known frequency stabilization approach controls some property of the laser (pump current, temperature, etc.) to keep the laser frequency/wavelength matched to a reference frequency or length. Some commonly used frequency reference sources include spectroscopic references such as molecular or atomic absorption and length references such as Fabry-Perot cavities or Michelson interferometers. However, one problem with these approaches is that the available reference frequencies are fixed.
Another known laboratory-based approach to frequency tuning modifies the frequency reference in order to tune the frequency of the resonant features. One example of this is adding a piezoelectric element to the spacer in a Fabry-Perot cavity. However, this approach generally has a detrimental effect on stability and reduces the noise performance of the stabilization system.
Yet another alternative approach stabilizes the laser to a fixed frequency reference and uses a frequency-tuning element, such as an acousto-optic device or phase-locked slave laser, to adjust the output laser frequency. This approach generally increases the complexity of the system by adding additional components that consume volume, mass and power. For example, acousto-optic devices in particular require large amounts of RF power. While additional volume, mass, and power are not necessarily of much concern in terrestrial applications, they are particularly valuable resources in space-based applications. In space-based applications, every additional kilogram requires a huge expense to launch to space and every additional watt requirement leads to additional mass in the form of batteries, solar panels, or other power-providing means. Accordingly, what is needed in the art is an improved, lower complexity, lower mass, more power efficient, tunable frequency-stabilized laser.